C3 revision

Easter holidays

Rates of reaction

Rates of reaction

• The rate of a reaction is the speed in which reactants are turned into products

• There are 4 ways you need to know about – what are they?

1. Temperature2. Surface area3. Concentration4. Using a catalyst

Surface area example

Giant statue 4 mini statues

They are made out of the same amount of limestone

Concentration example

Which box has the highest concentration of acid particles?

A B C

= water particle

= acid particle

High concentration

Low concentration

Medium concentration

0.2 M acid

0.5 M acid

1 M acid

Match the correct boxes

Extension – write word definitions of high and low concentration

Which 3 boxes would you choose to make the rate of reaction the fastest?

High temperature

Low surface area

Medium concentration

Medium temperature

High surface area

Low concentration

Low temperature

Medium surface area

High concentration

Helpful video

• http://www.youtube.com/watch?v=OttRV5ykP7A

Rate of reaction

The particles of the reactants need to get

together so that they can react

How does that work then?

Reactant particlescollide

Product particlesformed

REACTION

Some reactants have collision but with no

effectReactant particles

collide

Product particles not formed as there is

not enough energy

Activation energy

• Reactions only happen if the particles have enough energy

• The minimum amount of energy needed to start a reaction is called the Activation energy

• The amount of activation energy needed is different for each reaction

But

• Every reaction has activation energy, they all need a little push to get started.

More than just activation energy

C

O OCO O

More frequent successful collisions per second more product

The Collision Theory

• Particles are constantly moving

• For a chemical reaction to take place the reactant particles must collide first

• For the collision to be effective the particles must have the right amount of activation energy

• More collisions per second means that more product is made so the reaction is faster

Concentration

• More particles in the same space means more collisions• More successful collisions means more effective collisions• If we double the concentration we double the collisions

per second

Temperature

• Particles turn heat energy into kinetic energy• When they get hotter they move faster

• When they move faster they collide more often• More collisions per second means faster rate of reaction

Catalysts

• Catalysts reduce the activation energy needed for a reaction

• They do this by offering an alternate route for the reaction to take

• Less activation energy means more effective collisions• More frequent successful collisions per second means

faster rate

Activation energywith a catalyst

Surface Area/Particle Size

• Using smaller particles increases rate• Increase in surface area allows more collisions at

surface• More frequent successful collisions per second

means faster rate

What is the effect of reducing the concentration?

What is the effect of increasing concentration?

What is the effect increasing surface area?

What is the effect increasing the temperature?

What is the effect increasing the concentration?

Things to remember with rate graphs

• When graph plateaus the reaction is finished

• Faster reaction will be the line at a steeper gradient

• Reactions will ALWAYS stop because they will run out of reactants (limiting factor): could be one reactant or both

• We draw line of best fits on graphs so that we can– Extrapolate – make an estimate outside out results– Interpolate – make an estimate that is between our results

hydr

ogen

pro

duce

d (c

m3 )

time (seconds)10 20 30 40 50

10

20

30

40

50

60

70

00

x

y

Calculating rate of reaction from graphs

rate of reaction =

x

y

rate of reaction =

20 s

45 cm3 rate of reaction = 2.25 cm3/s

The gradient of the graph is equal to the initial rate of reaction at that time

How can the rate of reaction be calculated from a graph?

Calculations

Balancing equations

• Balance the below equations:

1.Mg + O2 MgO

2.AgNO3 + MgCl2 AgCl + Mg(NO3)2

ANSWERS1.2Mg + O2 2MgO

2.2AgNO3 + MgCl2 2AgCl + Mg(NO3)2

Balanced Equations

• They tell us how much of each substance is involved in a chemical reaction

2H2 + O2 2H2O

2 parts of hydrogen react with 1 part of oxygen to make 2 part of water

• This is useful because now we can use it to work out what mass of hydrogen and oxygen we need and how much water is made

2H2 + O2 2H2O

Masses in the equation

• To find out the masses in the equation, all we need to do is add up the masses of each individual compound in the equation…

• 2 parts of hydrogen = 2x 2g = 4g• 1 part of oxygen = 1 x 16g = 16g• 2 part of water = (2 x 1g)+ (2 x 16g) = 36g

Parts to moles

• When we have mentioned ‘parts’, in chemistry what we actually call it is moles

• 1 mole is the relative molecular mass of the compound

• Using your periodic table work out the RMM of the following:

H2 O2 MgSO4

CH4 H2O CuS

Activity• Balance the equations below• Write the number of moles of each we have• Work out the RMM of each component

1.Mg + HCl MgCl2 + H2

2.NaOH + Cl2 NaOCl + NaCl + H2O

EXTENSIONWrite what amount of MgCl2 we would make from 10g of

Mg

SUPPORTGrab a mini whiteboard

Answers

1. Mg + 2HCl MgCl2 + H2

1 mole of magnesium reacts with 2 moles of hydrochloric acid to produce 1 mole of magnesium chloride and 1 mole of hydrogen.

1 mole of Magnesium = 24g 1 mole of magnesium chloride = 95g2 moles of hydrochloric acid = 73g 1 mole of hydrogen = 2g

2. 2NaOH + Cl2 NaOCl + NaCl + H2O2 moles of sodium hydroxide reacts with 1 mole of chlorine to produce 1 mole

of sodium hypochlorite, 1 mole of sodium chloride and 1 mole of water

2 moles of sodium hydroxide = 80g 1 mole of sodium hypochloite = 74.5g1 mole of chlorine= 71g 1 mole of sodium chloride = 58.5g

1 mole of water = 18g

Conservation of mass

• What ever is used at the start of a reaction (reactant), has to be there at the end of the reaction (product)

• This is why we have to balance equation and why we can work out reacting masses

Some examples of questions1. Zinc oxide (ZnO) is heated with carbon (C) to make zinc

(Zn) and carbon monoxide (CO). How much zinc oxide do you need to make 130 tonnes of zinc?

2. A student adds 4.8g of magnesium (Mg) to excess dilute hydrochloric acid (HCl). Hydrogen is made but what mass of magnesium chloride (MgCl2) would be made?

3. If you add 5.5g of sodium carbonate (Na2CO3) to excess dilute sulphuric acid(H2SO4), what mass of sodium sulphate (NaSO4) would be made? Water and carbon dioxide is also made.

162 tonnes

19g

6.1g (Does not specify rounding/SF)

Worked answers

Percentage yield and atom economy

Percentage Yield

• Percentage yield is a way of comparing the amount of product made (actual yield) to the amount of product expected to be made (predicted yield).

• You can calculate percentage yield by using this formula:

Actual yield

Predicted yieldx 100

Percentage Yield Example

• A company making sulphuric acid gets an actual yield of 74 tonnes. They predicted a yield of 85 tonnes. What is the percentage yield?

Actual yield

Predicted yieldx 100

74

85= 87%

Percentage yieldFred reacted potassium iodide with lead nitrate. He filtered the solution to collect the products. He expected to make 20g but only made 18g.

1.What was his expected yield?2.What was his actual yield?3.What was his percentage yield?4.Suggest a reason for this loss of product5.What mass of product would he have made if his percentage yield was 72%?

Percentage Yield in Industry

• There are several reasons why the percentage yield is less than the expected yield.

• The products could be lost in evaporation, filtration or during the transfer of liquids.

• Not all reactants may have been used to make the products.

• In industrial processes a high percentage yield is desired• This reduces waste and therefore cost.

Atom economy

• Way of measuring the amount of waste in a reaction – measures loss of atoms

• Inefficient (wasteful) products have a low atom economy

• Efficient processes have a high atom economy

Atom Economy (%) = mass of desired producttotal mass of products

x 100

Example

Hydrogen is needed for the hydrogenation of oils to make margarine. Without the hydrogen, the margarine would not be solid. What is the atom economy for producing hydrogen from

reacting natural gas (methane) with steam?

CH4(g) + H2O(g) CO(g) + 3H2(g)

Step 1 – work out the relative atomic mass and relative molecular mass.

Ar – 12 + (4 x 1) (2 x 1) + 16 12 + 16 3 x (2 x 1)

Mr – 16 18 28 6

Step 2 – work out the mass of products and mass of desired products.

Total mass of products = 34

Mass of desired product (H2) = 6

Step 3 – calculate the atom economy using the formula.

Atom economy

• Every industry wants a high atom economy and a high percentage yield

• They want to make as much of the desired product as possibly and get as little waste as possible

• Reactions with low atom economy and % yield are inefficient and cost money

Processes to make chemicals• Chemicals are made through either batch or

continuous processes

• Batch – made to a certain recipe and many different types. Chemical/drug is made in batches and in small amounts

• Continuous – always the same recipe, huge amounts made on a continuous production 24/7

Exothermic and endothermic reactions

Endo and exothermic reactions

• Exothermic – heat is given out from the reaction (exit)

• Endothermic – heat is taken in to the reaction (enter)

• Both reactions involve a temperature change which can be measure with a thermometer

Endo and exothermic reactions

• When bonds break – energy is taken in (endo)

• When bonds are made – energy is given out (exo)

• What would a graph look like to show this? (think back to activation energy graphs when we were discussing catalysts)

Energy level diagrams

What type of reaction is shown on this graph?

Energy level diagrams

This is an endothermic reaction, energy is taken in from the environment around the reaction (reactants lower than products)

Reaction graph

Macromolecules

Graphite and diamond• Graphite and diamond are macromolecules• You need to know about their structure and their bonding

Graphite DiamondLustrous/black/opaque Lustrous/shiny/transparent/

colourlessSoft and slippery Very hardVery high melting point Very high melting pointInsoluble in water Insoluble in waterConducts electricity Does not conduct electricity

Diamond vs Graphite

• Both are made only from carbon

• Carbon is very good at bonding to itself and ideally each carbon atom wants to make 4 bonds (how do we know this?)

• Both are allotropes of carbon – elements in different forms

Diamond

• Diamond is formed when each carbon forms 4 bonds each to another carbon

• The structure around each atom is tetrahedral (4 sided) like a triangular based pyramid

• In 3D the bonds are as far apart from each other as possible

Giant covalent structure

• This gives diamonds a regular continuous structure and because each bond within it is very strong

• Diamond is the hardest naturally occurring substance, and used in drills and cutting tools

• It also means that the melting point is extremely high (about 38000C) as all the bonds need to be broken for the solid to melt

• There are NO free electrons in diamond so it does not conduct electricity

Graphite

• Graphite differs from carbon because each carbon atom only bonds to three others and not the usual four

• This means that there is an unbonded electron so graphite conducts electricity•The hexagons form into

flat sheets and there are weak interactions between each sheet

Giant covalent structure

• Because of the strong bonds between the atoms, graphite also has a high melting point but it is not just as high as diamond’s

• It is a relatively hard structure (because of the strong C-C bonds) but the interactions between each sheet are weak and so the layers can slide over each other

• In addition, the extra electron is free to move and so graphite can conduct electricity

Buckminster Fullerene

• Buckminster Fullerene – the bucky ball - is C60

• It is a sphere of carbon atoms bonded like the patches on a football

• Each carbon has three bonds, and some are in hexagons and others in pentagons

Giant covalent structure

• These have free electrons like graphite so conduct electricity

• Scientists have managed to join many bucky balls together to make a bucky tube – used in sport equipment to reinforce

• They are also used to trap/carry molecule. Researchers are looking into the being used to deliver drugs around the body

• Also used as industrial catalysts